Why is Network Render creating .exr's instead of .png's?
According to this bug report: 


  "the problem is that even if you have PNG selected in the blend to be
  rendered, if you use the 'Download render results from Network', they
  will always come back as layered .exr files. You only get them as PNGs
  when you press 'get animation' button."


Have you tried the Get Animation button instead?

Or, according to this page:


  If you just need PNG then in the network render properties, under
  client tab you should choose 'animation on network' rather than 'send
  job to network'. Both will render in EXR file but with 'animation on
  network' option, it'll deliver the png (or whatever format you choose)
  to specified folder. 


I would say, if you set your Render output folder path to "//something", which means relative to the blend file, with the output type as PNG, and then initiated the job by choosing Animation on Network, and then when complete, retrieve the frames with the Get Animation button, it should bring back PNG files.